Display devices for use in TVs, cell phones, etc., and optical elements, such as camera lenses, etc., usually adopt an antireflection technique in order to reduce the surface reflection and increase the amount of light transmitted therethrough. This is because, when light is transmitted through the interface between media of different refractive indices, e.g., when light is incident on the interface between air and glass, the amount of transmitted light decreases due to, for example, Fresnel reflection, thus deteriorating the visibility.
An antireflection technique which has been receiving attention in recent years is forming over a substrate surface a very small uneven pattern in which the interval of recessed portions or raised portions is not more than the wavelength of visible light (λ=380 nm to 780 nm). See Patent Documents 1 to 3. The two-dimensional size of a raised portion of an uneven pattern which performs an antireflection function is not less than 10 nm and less than 500 nm.
This method utilizes the principles of a so-called motheye structure. The refractive index for light that is incident on the substrate is continuously changed along the depth direction of the recessed portions or raised portions, from the refractive index of a medium on which the light is incident to the refractive index of the substrate, whereby reflection of a wavelength band that is subject to antireflection is prevented.
The motheye structure is advantageous in that it is capable of performing an antireflection function with small incident angle dependence over a wide wavelength band, as well as that it is applicable to a number of materials, and that an uneven pattern can be directly formed in a substrate. As such, a high-performance antireflection film (or antireflection surface) can be provided at a low cost.
As the method of forming a motheye structure, using an anodized porous alumina which is obtained by means of anodization (or “anodic oxidation”) of aluminum has been receiving attention (Patent Documents 2 and 3 and Non-patent Document 1).
Now, the anodized porous alumina which is obtained by means of anodization of aluminum is briefly described. Conventionally, a method of forming a porous structure by means of anodization has been receiving attention as a simple method for making nanometer-scale micropores (recessed portions) in the shape of a circular column in a regular arrangement. A base is immersed in an acidic electrolytic solution of sulfuric acid, oxalic acid, phosphoric acid, or the like, or an alkaline electrolytic solution, and this is used as an anode in application of a voltage, which causes oxidation and dissolution. The oxidation and the dissolution concurrently advance over a surface of the base to form an oxide film which has micropores over its surface. The micropores, which are in the shape of a circular column, are oriented vertical to the oxide film and exhibit a self-organized regularity under certain conditions (voltage, electrolyte type, temperature, etc.). Thus, this anodized porous alumina is expected to be applied to a wide variety of functional materials.
An anodized porous alumina layer 40 is constituted of cells 46 of equal sizes, each of which has a micropore 42 and a barrier layer 44, as schematically shown in FIG. 11. In the porous alumina layer 40 fabricated under specific conditions, the schematic shape of the cells 46 when seen in a direction perpendicular to the film surface is a generally regular hexagon. The cells 46 are in a two-dimensionally closest packed arrangement when seen in a direction perpendicular to the film surface. Each of the cells 46 has a micropore 42 at its center. The arrangement of the micropores 42 is periodic. The periodic arrangement of the micropores 42 herein means that, when seen in a direction perpendicular to the film surface, the sum of vectors extending from the geometric centroid (hereinafter, simply referred to as “centroid”) of one micropore to the centroids of its neighboring micropores (vector sum) is zero. In the example shown in FIG. 11, 6 vectors extending from the centroid of one micropore 42 to the centroids of 6 neighboring micropores 42 have equal lengths, and the directions of the vectors are different by the angles of 60°. Thus, the sum of these vectors is zero. In an actual porous alumina layer, the micropores 42 are recognized as being periodic when the length of the sum of the vectors is less than 5% of the shortest one of the vectors.
The porous alumina layer 40 is formed by anodizing an aluminum surface and is therefore formed on an aluminum layer 48. The cells 46 are formed as a result of local dissolution and growth of a coating. The dissolution and growth of the coating concurrently advance at the bottom of the micropores which is referred to as a barrier layer 44. As known, the size of the cells 46, i.e., the interval between adjacent micropores 42, is approximately twice the thickness of the barrier layer 44, and is approximately proportional to the voltage that is applied during the anodization. It is also known that the diameter of the micropores 42 depends on the type, concentration, temperature, etc., of the electrolytic solution but is, usually, about ⅓ of the size of the cells 46 (the length of the longest diagonal of the cell 46 when seen in a direction vertical to the film surface). Such micropores 42 of the porous alumina may constitute an arrangement which has a high regularity (periodicity) under specific conditions, an arrangement with a regularity degraded to some extent depending on the conditions, or an irregular (non-periodic) arrangement.
Patent Document 2 discloses a method of producing an antireflection film (antireflection surface) by a transfer method with the use of a stamper which has an anodized alumina film over its surface.
Recently, Non-patent Document 1 disclosed that pieces of anodized alumina having various shapes were fabricated by repeating anodization of aluminum and a pore diameter increasing process. Non-patent Document 1 also disclosed that an alumina piece which had non-bellshape tapered micropores was used as a mold to fabricate an antireflection film of PMMA which had a motheye structure. The reflectance of this antireflection film is about 1% or less. The lateral surface of a recessed portion formed in an alumina layer described in Non-patent Document 1 is smooth (continuous) and linear.
The present applicant disclosed in Patent Document 3 that, by forming an antireflection film with the use of an alumina layer in which minute recessed portions have stepped lateral surfaces, the probability of reflection (0th-order diffraction) of light can be reduced as compared with the case where the alumina layer described in Non-patent Document 1 was used. Patent Document 3 also discloses that the stamper in which minute recessed portions have stepped lateral surfaces has a greater specific surface, and hence provides a greater surface treatment effect, than the stamper described in Non-patent Document 1. For example, the surface of the stamper may be subjected to a mold release treatment so that the transferability can be improved. Also, the surface of the antireflection film may be subjected to a water-repellent or oil-repellent treatment (e.g., fluoric treatment) so that a soil-release effect can be obtained.
As described in Patent Documents 1 to 3, by providing an uneven structure (macro structure) which is greater than a motheye structure (micro structure) in addition to the motheye structure, the antireflection film (antireflection surface) can be provided with an antiglare function. The size of a raised portion of the uneven structure which is capable of performing the antiglare function is not less than 1 μm and less than 100 μm. The entire disclosures of Patent Documents 1 to 3 are herein incorporated by reference.